1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for maintaining speech quality in a wireless network, and specifically to quantifying the degree of balance, and thus the speech quality, on both the forward and reverse links.
2. Background and Objects of the Present Invention
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications ever. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. Cellular networks have evolved in two different networks. The European cellular network uses the Global System for Mobile Communication (GSM) digital mobile cellular radio system. In the United States, cellular networks have traditionally been primarily analog, but recent advances have been incorporating digital systems within the analog networks. One such North American cellular network is the D-AMPS network, which is described hereinbelow.
With reference now to FIG. 1 of the drawings, there is illustrated a D-AMPS Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a Mobile Station (MS) 20 may move freely without having to send update location information to the MSC/VLR area 12 that controls the LA 18.
Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless. The MS 20 may also include a Subscriber Identity Module (SIM) card 13, or other memory, which provides storage of subscriber related information, such as a subscriber authentication key, temporary network data, and service related data (e.g. language preference).
Each Location Area 12 is divided into a number of cells 22. The MSC 14 is in communication with a Base Station (BS) 24, which is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible. The radio interface between the BS 24 and the MS 20 utilizes Time Division Multiple Access (TDMA) to transmit information between the BS 24 and the MS 20, with one TDMA frame per carrier frequency. Each frame consists of eight timeslots or physical channels. Depending upon the kind of information sent, different types of logical channels can be mapped onto the physical channels. For example, speech is sent on the logical channel, "Traffic Channel" (TCH), and signaling information is sent on the logical channel, "Control Channel" (CCH).
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the Mobile Stations 20 currently located within the MSC/VLR area 12. If an MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that MS 20 from the home HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
Currently, speech and data are transmitted from the BS 24 to the MS 20 on a forward link channel 30 and from the MS 20 to the BS 24 on a reverse link channel 32. Forward 30 and reverse 32 link speech quality balance is an important issue in mobile communications. An important design criterion in cellular systems 10 is that the quality on both links 30 and 32 should be the same. A perceivable difference in speech quality on the two links 30 and 32 can lead to customer dissatisfaction. Therefore, such an analysis is crucial for noise as well as interference limited systems.
The speech quality in digital cellular systems 10 is measured via quantities such as frame erasure, which is the percentage of TDMA frames that cannot be perceived, and the bit error rate (BER), which is an estimate of the number of coded bits in error. In order to measure the BER, the encoded bits that are transmitted in each burst or frame of data across the forward 30 or reverse 32 link channel are received by a receiver (not shown) and decoded, using, for example, a convolutional decoding algorithm. The algorithm also estimates how many errors were induced by the channel. This estimate of the BER can be referred to as the raw BER. It should be understood that the number of errors estimated by the convolutional decoder is just an estimate of the actual BER. However, this estimate can be considered reliable to a certain degree, and since convolutional codes are usually the most efficient coding mechanisms employed, the BER can be considered as the best estimate of the deterioration in speech quality.
Currently, the BER can be mapped to a particular BER class, which varies for different standards. The corresponding BER percentages for D-AMPS (IS-136) as well as Global System for Mobile Communications (GSM) is shown in Table 1 hereinbelow, for the eight BER classes (0-7).
TABLE 1 ______________________________________ Mapping the Signal Quality to the BER BER Class BER (%) for D-AMPS BER (%) FOR GSM ______________________________________ 0 Below 0.01 Below 0.2 1 0.01-0.1 0.2-0.4 2 0.1-0.5 0.4-0.8 3 0.5-1.0 0.8-1.6 4 1.0-2.0 1.6-3.2 5 2.0-4.0 3.2-6.4 6 4.0-8.0 6.4-12.8 7 Above 8.0 Above 12.8 ______________________________________
The raw bit error rate (BER) is quantized above into eight discrete levels or classes. The raw BER and BER class are integral for assessing the speech quality. The advantage of the actual BER percentage is that it is a relatively better metric for evaluating speech quality in comparison to the BER class. Compressing the information into classes results in a loss of information which makes this procedure inappropriate for use because the BER classes are on a non-linear scale. Therefore, the difference between class 1 and 2 may not be perceivable to the user. On the other hand, the difference between class 4 and 5 (2.5% BER vs 7.5% BER) is quite drastic. However, the BER class does give a concise and clear description of the speech quality to the system designer.
The BER on the forward 30 and reverse 32 links needs to be balanced, e.g., substantially equal, in order for both the calling party and the called party to perceive substantially equivalent voice quality. In many instances, the BER is not substantially equal on the forward 30 and reverse 32 links. For example, the BS 24 typically has two receiver antennas, for diversity, and one transmitting antenna. In certain areas of the cell 22, the reception on the forward link 30 can be poor, e.g., the bit error rate (BER) is high, because the transmitting antenna is not suitably located for this area of the cell 22, but, at the same time, the reception on the reverse link 32 can be good, e.g., the BER is low, because at least one of the receiving antennas is located satisfactorily with respect to the same area of the cell 22. Therefore, in order to maintain a system with balance links 30 and 32, the BER on both the forward link 30 and the reverse link 32 must be analyzed at each point in the cell 22.
One such method of analyzing the link balance is the link budget. The link budget allows the computation of the maximum tolerable path loss based upon the transmit power of the BS 24, P.sub.BS, the receiver sensitivity of the BS 24, S.sub.BS, the transmit power of the MS 20, P.sub.MS the receiver sensitivity of the MS 20, S.sub.MS, and the diversity gain G.sub.div. The transmit power for the BS 24 can be obtained from the system vendor, e.g., the performance characteristics of the equipment. The remaining parameters are obtained from the system specification document. In order to insure the same speech quality on both links 30 and 32, the maximum allowable path loss on the reverse link 32 should be the same as the maximum allowable path loss on the forward link 30. The maximum allowable path loss can be computed by taking into account the maximum transmit power and receiver sensitivity of the BS 24 and the MS 20. On the forward link 30, it is: EQU .vertline.PL.vertline..sub.FL =P.sub.BS -L.sub.f +G.sub.BS -S.sub.MS +G.sub.MS [ 1]
Similarly, on the reverse link 32, the maximum path loss that the system 10 can allow is: EQU .vertline.PL.vertline..sub.RL =P.sub.MS +G.sub.MS -S.sub.BS -G.sub.div -L.sub.f +G.sub.BS, [2]
where G.sub.BS and G.sub.MS are the antenna gains for the BS 24 and MS 20, respectively. For a balanced system, the path loss is balanced by taking the minimum of the maximum allowable path loss on the forward 30 and reverse 32 links, e.g., PL=min(.vertline.PL.vertline..sub.FL, .vertline.PL.vertline..sub.RL. Therefore, the path balance equation after canceling terms is: EQU P.sub.BS -S.sub.MS =P.sub.MS -S.sub.PS -G.sub.div. [3]
What Equation 3 above implies is that the power of the BS 24 has to be adjusted such that .vertline.PL.vertline..sub.FL =.vertline.PL.vertline..sub.RL, e.g., the path loss on the forward link 30 and the reverse link 32 are substantially the same. It should be noted that the above equation is true only for a noise-limited situation. If interference is dominant in the system, then equation [3] is no longer valid for path balance. Usually the forward link 30 is more prone to interference problems than the reverse link 32 because the BS 24 is transmitting on all timeslots. Therefore, balance speech quality is a key issue for cellular systems 10 and the speech quality balance can change drastically as the interference level fluctuates. Thus, it is important to notice this variation and adaptively update the cell 22 parameters/features, such that the speech quality balance is maintained.
As stated hereinbefore, usually the transmit power of the BS 24 is adjusted to maintain path balance. If the adjustment requires a decrease in the transmit power of the BS 24, that can be done with ease. However, great care is taken before the transmit power is increased, as that can also lead to greater co-channel/multiple access interference for TDMA/CDMA systems. Once the system designer has adjusted these parameters, the goal is to assess if the speech quality is balanced on the forward 30 and reverse 32 links.
The traditional approach for path balance does not take into account the interference levels on both links. One reason is that unlike measuring reverse link interference, it has not been traditionally possible for the system engineer to measure the forward link interference. Hence, the engineer is not able to balance the two links in the appropriate manner. Usually, the engineer utilizes equation [3] above or a similar approach, which assumes that the system noise is limited. However, this approach is suboptimal as it disregards the interference levels on the two links. Furthermore, this existing approach does not enable the engineer to have the ability to statistically analyze the degree of balance on the forward and reverse links.
Another traditional technique of assessing the speech quality balance includes plotting the speech quality on the forward 30 and reverse 32 links as a function of time. However, this type of graph cannot yield meaningful information, because it is the statistics of the speech quality which is of importance to the cellular network designer. This is due to the fact that the forward 30 and reverse 32 links are subject to independent short-term fading because the forward 30 and reverse 32 links are on two separate radio frequencies. As a result, the BER on the two links 30 and 32 is independent as far as the short-term fading is concerned. Therefore, the only accurate way to assess the speech quality balance is to perform a statistical analysis of the information.
Yet another known method of comparing the speech quality on the forward 30 and reverse 32 links is to compare the Cumulative Distribution (CDF) for the forward 30 and reverse 32 link voice quality, as shown in FIG. 2. As can be seen, for the example shown in FIG. 2, the reverse link 32 has a higher proportion of lower BER class measurements, indicating better performance on that link 32, e.g., the system is limited as far as the forward link 30 is concerned. However, the degree of this limitation is not easy to quantify by visual inspection of the CDF. Thus, distinguishing between a balanced and unbalanced system can be tricky. The CDF only gives partial information, and therefore, the degree of the balance is not clear with great statistical confidence.
It is, therefore, an object of the invention to statistically compare the speech quality, e.g., the BER, on the forward and reverse links in order to quantify the degree of balance on the links based upon the statistical significance of the data.
It is a further object of the invention to benchmark the BER balance on the forward and reverse links as a "statistic", which can be used for comparison purposes or in other scenarios, such as when the cellular system is sufficiently loaded and is now interference limited.
It is still a further object of the present invention to develop a methodology to balance an interference limited system.
It is still a further object of the present invention to substantially continuously adjust the power levels on the forward and reverse links in an adaptive manner in accordance with balance measurements to maintain balance on the forward and reverse links.